Purpose:
Previous studies have determined the quantity and total enzymatic activity of lysozyme deposited on various contact lens materials. The purpose of this study was to determine and compare the levels of surface versus bulk active lysozyme deposited on several commercial contact lens materials.

Methods:
Six different hydrogel, daily disposable, contact lens materials [polymacon, omafilcon A, nelfilcon A, nesofilcon A, ocufilcon and etafilcon A with polyvinylpyrrolidone (PVP)] were incubated in a complex artificial tear solution containing lysozyme for 16 hours. After the specified incubation period, total activity and surface activity of the deposited lysozyme were determined using two different techniques. Total activity was determined by extracting lysozyme and quantifying the activity using a standard turbidity assay. The surface activity of the deposited lysozyme was determined using a modified turbidity assay. The amount of active lysozyme present within the bulk of the lens material was calculated by determining the difference between the total and surface active lysozyme.

Results:
The etafilcon A material showed the highest amount of total lysozyme activity (517±1 µg/lens), followed by the ocufilcon material (200±6 µg/lens) and these two were significantly different from each other (p<0.05). All other lenses deposited <6 µg of total active lysozyme per lens. The amount of surface active lysozyme on etafilcon and ocufilcon lens materials was significantly higher than that found on all other lenses (p<0.05). There was no active lysozyme quantified in the bulk of the nelfilcon material, as all of the active lysozyme was found on the surface (1.7±0.3µg/lens). In contrast, no active lysozyme was quantified on the surface of polymacon, with all of the active lysozyme found in the bulk of the lens material (0.6±0.6 µg/lens). The activity of lysozyme on the surfaces of nesofilcon, nelfilcon and omafilcon were 2.5±0.6µg/lens, 1.7±0.2 µg/lens and 2.5±0.8 µg/lens respectively and were not significantly different from each other (p>0.05).

Conclusions:
The surface and bulk activity of lysozyme deposited on contact lenses varies depending on the water content, ionicity and the polymeric make-up of the material. Lysozyme deposited on ionic and high water content lens materials such as etafilcon A show significantly higher surface and bulk activity than several other tested lens materials.